What does "Unreachable" below my light names mean?

There is nothing wrong but it seems that your Hue light has been physically switched off (e.g. by using the wall switch) or is out of the operating range of the system. If the text pops up when other wireless devices are used, like WiFi camera, streaming video (watching TV on your device) or streaming audio and the hue system becomes uncontrollable, perform a ZigBee Channel Change. Go to 'Settings' > 'Hue bridges' > tap on "i' > 'Zigbee channel change' and follow onscreen instructions. For further troubleshooting contact our customer support.